RFID tags with shielding structure for incorporation into microwavable food packaging

ABSTRACT

RFID tags are provided for incorporation into the packaging of a microwavable food item, with the RFID tag being configured to be safely microwaved. The RFID tag includes an antenna defining a gap and configured to operate at a first frequency. An RFID chip is electrically coupled to the antenna across the gap. A shielding structure is electrically coupled to the antenna across the gap and overlays the RFID chip. The shielding structure includes a shield conductor and a shield dielectric at least partially positioned between the shield conductor and the RFID chip. The shielding structure is configured to limit the voltage across the gap when the antenna is exposed to a second frequency that is greater than first frequency. In additional embodiments, RFID tags are provided for incorporation into the packaging of a microwavable food item, with the RFID tag being configured to be safely microwaved. The RFID tag includes an RFID chip and an antenna electrically coupled to the RFID chip. The antenna may have a sheet resistance in the range of approximately 100 ohms to approximately 230 ohms, optionally with an optical density in the range of approximately 0.18 to approximately 0.29. Alternatively, or additionally, the antenna may be configured to fracture into multiple pieces upon being subjected to heating in a microwave oven. Alternatively, or additionally, the RFID tag may be incorporated in an RFID label that is secured to the package by a joinder material with a greater resistance than that of the antenna, such as a sheet resistance in the range of approximately 100 ohms to approximately 230 ohms.

CROSS REFERENCE TO RELATED APPLICATION(S)

The present application claims priority to and the benefit of U.S.Provisional Patent Application No. 62/440,108 filed Dec. 29, 2016 andU.S. Provisional Patent Application No. 62/539,817 filed Aug. 1, 2017,each of which is incorporated herein by reference in its entirety.

BACKGROUND Field of the Disclosure

The present subject matter relates to packaging for microwavable fooditems. More particularly, the present subject matter relates to radiofrequency identification (“RFID”) tags incorporated into packaging formicrowavable food items.

Description of Related Art

It is known for packages for microwavable food items to include cookingaids that are to be placed into the microwave oven with the food itemfor cooking/heating the food item. For example, foods having crusts,such as frozen pies or stuffed bread, may benefit from “crispingsleeves,” which are paper items that at least partially surround thefood item during microwaving. Typically, a “crisping sleeve” has a papersubstrate, with a susceptor incorporated into the inner surface of the“crisping sleeve,” facing and preferably in contact with the food item.The susceptor, which may be a metallized film, absorbs microwave energyand converts it into heat, which crisps and/or browns the crust orsurface of the food item, thus improving the look and texture of thefood item. Due to the absorbing nature of the film used as thesusceptor, relatively low levels of energy are reflected by it, suchthat it does not strike an arc due to generating high differentialvoltages between adjacent parts of the film, which could otherwise causethe packaging to catch fire.

It is also known to incorporate RFID technology, such as an RFID tag,into product packaging for various purposes, including inventorymanagement and theft prevention. FIG. 1 shows an RFID tag T according toconventional design, which may be secured to or otherwise associatedwith an enclosure like that of enclosure 13 of FIG. 1A (typically, apaper or cardboard sleeve or box) of the package 9 for a microwavablefood item in respect to FIG. 1A. The entirety of the packaging 9 of FIG.1A is not intended to be microwaved, but rather the food item (and,optionally, a “crisping sleeve” or the like) is removed from theenclosure 13 of FIG. 1A and inserted into the microwave oven forheating/cooking.

The RFID tag T of FIG. 1 includes an RFID chip C, with an associateddipole antenna A for transmitting information to and/or receivinginformation from an RFID reader (not illustrated). The RFID chip C iselectrically coupled to the antenna A across a gap G defined by theantenna A between two conductor pad areas P.

RFID tags inherently must, at some point, have a gap across which theRFID chip is placed that has a voltage at the intended frequency ofoperation when in the field of a reader device. The power requiredincident on the RFID chip C may be as low as 10 microwatts, whereas amicrowave oven may typically operate at a power level in excess of 800watts, which can generate very high voltages across the gap G and theassociated RFID chip C. The antenna A is designed to operate at a firstfrequency F1, for example in the range of approximately 860 MHz to 930MHz, with the antenna A taking incident power at the first frequency F1from an RFID reader and converting it to a voltage across the RFID chipC to allow it to operate.

A second frequency applied by the microwave oven, identified in FIG. 1at F2, typically on the order of approximately 2,450 MHz, may also beincident on the antenna A when the RFID tag T is placed into themicrowave oven. The antenna A is not designed to operate at the secondfrequency F2, as the very high power levels incident at second frequencyF2 will generate high voltages on the antenna A. These high voltages canappear at a number of places on the antenna A; however, by methods suchas introducing large gaps L between antenna elements and controlledradii (identified generally at R in FIG. 1), a voltage across saidelements that would generate a high voltage breakdown and, hence, arccan be avoided. However, the gap G bridged by the RFID chip C isnecessarily relatively small and, hence, a high voltage arises at thesecond frequency F2, which high voltage may cause a breakdown andgenerate an arc. Similarly depicted in FIG. 1A; the dipole antenna 17can receive microwave energy (identified in FIG. 1A at M) and reflectthe microwave energy (represented in FIG. 1A at R) into the microwavesource. There is the possibility that an arc may be created betweenadjacent sections of the dipole antenna 17 (which location may bebetween the two conductive elements of the dipole antenna 17, asidentified in FIG. 1A at 19). Additionally, referring to FIG. 1A thedipole antenna 17 of the conventional RFID tag 11 is formed ofrelatively thick, low resistance conductor, which has differentproperties than the metallic film used to define a typical susceptor.For example, common susceptors are made from metal-coated films withoptical densities ranging from 0.18 to 0.29, corresponding to a sheetresistance of 100 ohms to 230 ohms, whereas a material of less than 1ohm per square is commonly used to form the antenna 18 of the RFID tag11. On account of the characteristics of the dipole antenna 17, the RFIDtag 11 can cause issues if it is not dissociated from the food itemprior to microwaving the food item (i.e., if the entire package 9 ofFIG. 1A is placed into the microwave oven).

To avoid problems of this nature, the RFID tag T and 11 of FIGS. 1 and1A respectively, are typically configured to be readily removable orotherwise dissociable from the food item, such as by securing it to theenclosure of the package, which may include instructions to notmicrowave the enclosure. However, it is possible that a user failing totake proper care could place the entire package (including the RFID tagT and 11 shown in FIGS. 1 and 1A respectively) into the microwave ovenwith the food item, thereby failing to dissociate the RFID tag T or 11from the food item. Accordingly, it would be advantageous to provide anRFID tag that may be microwaved without resulting in the problemsassociated with microwaving a conventional RFID tag T or 11.

SUMMARY

There are several aspects of the present subject matter which may beembodied separately or together in the devices and systems described andclaimed below. These aspects may be employed alone or in combinationwith other aspects of the subject matter described herein, and thedescription of these aspects together is not intended to preclude theuse of these aspects separately or the claiming of such aspectsseparately or in different combinations as may be set forth in theclaims appended hereto.

In one aspect, an RFID tag includes an antenna defining a gap andconfigured to operate at a first frequency. An RFID chip and an antennaelectrically coupled to the antenna across the gap. A shieldingstructure is electrically coupled to the antenna across the gap andoverlays the RFID chip. The shielding structure includes a shieldconductor and a shield dielectric at least partially positioned betweenthe shield conductor and the RFID chip. The shielding structure isconfigured to limit the voltage across the gap when the antenna isexposed to a second frequency that is greater than the first frequency.

In another aspect, packaging is provided for a microwavable food item.The packaging includes an enclosure and an RFID tag secured to theenclosure. The RFID tag includes an antenna defining a gap andconfigured to operate at a first frequency. An RFID chip is electricallycoupled to the antenna across the gap. A shielding structure iselectrically coupled to the antenna across the gap and overlays the RFIDchip. The shielding structure includes a shield conductor and a shielddielectric at least partially positioned between the shield conductorand the RFID chip. The shielding structure is configured to limit thevoltage across the gap when the antenna is exposed to a second frequencythat is greater than the first frequency.

In a further aspect, an RFID tag includes an antenna defining a gap andconfigured to operate at a first frequency. An RFID chip is electricallycoupled to the antenna across the gap. A shielding structure iselectrically coupled to the antenna across the gap and overlays the RFIDchip. The shielding structure includes a shield conductor and a shielddielectric at least partially positioned between the shield conductorand the RFID chip. A second shielding structure is electrically coupledto the antenna across the gap, underlying the RFID chip. The shieldingstructure is configured to limit the voltage across the gap when theantenna is exposed to a second frequency that is greater than the firstfrequency.

In another aspect, packaging is provided for a microwavable food item.The packaging includes an enclosure and an RFID tag secured to theenclosure. The RFID tag includes an antenna defining a gap andconfigured to operate at a first frequency. An RFID chip is electricallycoupled to the antenna across the gap. A shielding structure iselectrically coupled to the antenna across the gap and overlays the RFIDchip. The shielding structure includes a shield conductor and a shielddielectric at least partially positioned between the shield conductorand the RFID chip. A second shielding structure is electrically coupledto the antenna across the gap, underlying the RFID chip. The shieldingstructure is configured to limit the voltage across the gap when theantenna is exposed to a second frequency that is greater than the firstfrequency.

In another aspect the antenna comprised of an antenna with a sheetresistance in the range of approximately 100 ohms to approximately 230ohms. In another aspect, an RFID tag includes an RFID chip and anantenna electrically coupled to the RFID chip. The antenna is comprisedof a conductor formed of a base material and a second material withdifferent coefficients of thermal expansion configured to cause theantenna to fracture into multiple pieces upon being subjected toheating.

In yet another aspect, a package is provided for a microwavable fooditem. The package includes an enclosure, an RFID label, and a joindermaterial sandwiched between the RFID label and the enclosure. The RFIDlabel includes a substrate and an RFID tag associated with thesubstrate. The RFID tag includes an RFID chip and an antennaelectrically coupled to the RFID chip. The joinder material has agreater resistance than the antenna.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a top plan view of an RFID tag according to conventionaldesign;

FIG. 1A is a perspective view of a package for a microwavable food itemincorporating an RFID tag according to conventional design;

FIG. 2A is a top plan view of an RFID tag according to aspects of thepresent disclosure, which may be incorporated into packaging for amicrowavable food item;

FIG. 2B is a cross-sectional side view of a portion of the RFID tag ofFIG. 2A, secured to packaging for a microwavable food item;

FIG. 3A is a top plan view of another embodiment of an RFID tagaccording to aspects of the present disclosure, which may beincorporated into packaging for a microwavable food item;

FIG. 3B is a cross-sectional side view of a portion of the RFID tag ofFIG. 3A, secured to packaging for a microwavable food item;

FIG. 4A is a top plan view of a third embodiment of an RFID tagaccording to aspects of the present disclosure, which may beincorporated into packaging for a microwavable food item;

FIG. 4B is a cross-sectional side view of a portion of the RFID tag ofFIG. 4A;

FIG. 5 is a top plan view of a fourth embodiment of an RFID tagaccording to aspects of the present disclosure, which may beincorporated into packaging for a microwavable food item;

FIG. 6A is a top plan view of a fifth embodiment of an RFID tagaccording to aspects of the present disclosure, which may beincorporated into packaging for a microwavable food item;

FIG. 6B is a cross-sectional side view of a portion of the RFID tag ofFIG. 6A, secured to packaging for a microwavable food item;

FIG. 7A is a top plan view of a sixth embodiment of an RFID tagaccording to aspects of the present disclosure, which may beincorporated into packaging for a microwavable food item;

FIG. 7B is a cross-sectional side view of a portion of the RFID tag ofFIG. 7A; and

FIG. 8 illustrates a basic equivalent circuit of a portion of an RFIDtag according to aspects of the present disclosure.

FIG. 9 is a perspective view of a package for a microwavable food itemincorporating an RFID tag according to aspects of the presentdisclosure;

FIG. 10 is a top plan view of an alternative embodiment of an antenna ofan RFID tag according to aspects of the present disclosure, which may beincorporated into a package for a microwavable food item;

FIG. 10A is a top plan view of the antenna of FIG. 10 following heating;and

FIG. 11 is an exploded, perspective view of an alternative embodiment ofa package for a microwavable food item incorporating an RFID tagaccording to aspects of the present disclosure.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention, which may be embodied in variousforms. Therefore, specific details disclosed herein are not to beinterpreted as limiting, but merely as a basis for the claims and as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention in virtually any appropriate manner.

FIGS. 2A and 2B show an RFID tag 10 according to the present disclosure,while FIG. 2B shows the RFID tag, generally designated at 10, secured tothe enclosure 12 (e.g., a paper box) of packaging, generally designatedat 14, for a microwavable food item. The packaging 14 may include otheritems, such as a “crisping sleeve” configured to be microwaved with thefood item. The RFID tag 10 may be incorporated into the packaging 14 byany suitable approach and, while the RFID tag 10 is secured to theenclosure 12 in the embodiment of FIG. 2B, the RFID tag 10 may beassociated with another portion of the packaging 14 (e.g., a “crispingsleeve” housed within the enclosure 12) in other embodiments. Further,while RFID tags are described herein as being incorporated into thepackaging of a microwavable food item, it should be understood that RFIDtags according to the present disclosure may be useful in any of anumber of possible applications, particularly when it is contemplatedthat they may be exposed to frequencies (referred to herein as a “secondfrequency”) that are significantly higher than the frequency (referredto herein as a “first frequency”) at which an antenna of the RFID tag isintended to operate.

The RFID tag 10 includes an antenna 16 with an RFID chip 18 electricallycoupled thereto. The antenna 16 is provided as a dipole antenna, whichis formed of a conductor defining a gap 20 between two conductor padareas 22 (FIG. 2A), which is bridged by the RFID chip 18. The antenna 16and RFID chip 18 may be provided generally according to conventionaldesign (e.g., as described above with respect to the embodiment of FIG.1), with the antenna 16 being designed to operate at a first frequency,which may be in the range of approximately 860 MHz to 930 MHz. As in theconventional RFID tag T, the antenna 16 takes incident power at thefirst frequency and converts it to a voltage across the RFID chip 18 toallow it to operate.

The RFID chip 18 may take any of a number of forms (including those ofthe type commonly referred to as a “chip” or a “strap” by one ofordinary skill in the art), including any of a number of possiblecomponents and being configured to perform any of a number of possiblefunctions. For example, in one embodiment, the RFID chip 18 includes anintegrated circuit for controlling RF communication and other functionsof the RFID tag 10.

The RFID tag 10 further includes a shielding structure, generallydesignated at 24, which is comprised of a shield conductor 26 and ashield dielectric 28. The shield conductor 26 is formed of a materialhaving conductive properties and, as will be described in greaterdetail, may be variously configured without departing from the scope ofthe present disclosure. The shield dielectric 28 is formed of a materialhaving dielectric properties and, as will be described in greaterdetail, may be variously configured without departing from the scope ofthe present disclosure. For example, in the embodiment of FIGS. 2A and2B, the shield conductor 26 and shield dielectric 28 are generally flator planar, substantially identically shaped, and oriented with theperimeter of the shield conductor 26 coinciding with the perimeter ofthe shield dielectric 28. In other embodiments, the shield conductor andshield dielectric may be differently configured and/or oriented at leastpartially out of alignment (i.e., with a portion of the shield conductorextending beyond the perimeter of the shield dielectric and/or a portionof the shield dielectric extending beyond the perimeter of the shieldconductor).

The shielding structure 24 is electrically coupled to the antenna 16across the gap 20, being coupled by capacitance to the conductor padareas 22 on either side of the gap 20 (FIG. 2A). As shown in FIG. 2B,the shielding structure 24 overlays the RFID chip 18, with the shielddielectric 28 at least partially positioned between the RFID chip 18 andthe shield conductor 26. The shielding structure 24 may overlay or coverall (as in FIGS. 2A and 2B) or only a portion of the gap 20.

As described above, it is possible for the RFID tag 10 to be exposed tosignals operating at first or second frequencies. When the RFID tag 10is exposed to the first frequency, the shielding structure 24 forms apartial short circuit across the gap 20. However, the antenna 16 isconfigured so as to compensate for the presence of the partial shortcircuit, thereby allowing the RFID tag 10 to operate properly.

As described above, when a conventional RFID tag T is exposed to thesecond frequency F2, a large voltage arises across the gap G, whichrisks the creation of an arc. If the voltage and power at the secondfrequency F2 are limited sufficiently, the RFID chip C may survive, butthe main objective is to prevent an arc that could ignite the RFID tag Tor the packaging 14 into which it is incorporated. The shieldingstructure 24 of FIGS. 2A and 2B provides this function by “shorting” thehigh voltage generated across the gap 20 (and, hence, the RFID chip 18)when the RFID tag 10 is exposed to the second frequency, therebyreducing the voltage below the level that can cause a breakdown andpossible arc, which prevents ignition. Accordingly, the RFID tag 10 maybe placed into a microwave and exposed to the attendant high-frequencysignals (which may be on the order of approximately 2,450 MHz) withoutthe risk of ignition, unlike a conventional RFID tag T.

The shielding structure may be variously configured without departingfrom the scope of the present disclosure, as noted above. For example,FIGS. 3A and 3B show an embodiment of an RFID tag, generally designatedat 10 a, (and associated packaging, generally designated at 14 a, inFIG. 3B) in which the shielding structure 24 a includes a differentlyconfigured shield dielectric 28 a (FIG. 3B). In the embodiment of FIGS.3A and 3B, the shield dielectric 28 a is incorporated into anover-lamination layer, which overlays the RFID chip 18, at least aportion of the gap 20, and at least a portion of the conductor pad areas22 of the antenna 16 (FIG. 3A). The shield conductor 26 a may comprise apatterned conductor to provide the desired bridging and shieldingeffects. As best seen in FIG. 3B, the shield conductor 26 a and shielddielectric 28 a may be differently sized and shaped, with the shieldconductor 26 a being smaller than the over-lamination layer into whichthe shield dielectric 28 a is incorporated.

FIGS. 4A and 4B illustrate another embodiment of an RFID tag, generallydesignated at 10 b, according to the present disclosure. In theembodiment of FIGS. 4A and 4B, the shielding structure, generallydesignated at 24 b, is incorporated into an RFID strap comprised of astrap conductor 30 and strap substrate 32 (along with the RFID chip 18),which is electrically coupled to the antenna 16, across the gap 20. Theshielding structure 24 b may be comprised of a shield conductor 26 bapplied to the strap substrate 32, which serves as the shield dielectric28 b. The strap substrate 32 (and any of the other shield dielectricsdescribed herein) may be formed of any of a variety of materials, suchas polyethylene terephthalate.

FIG. 5 illustrates another embodiment of an RFID tag, generallydesignated at 10 c, with a differently configured shielding structure 24c. In the embodiment of FIG. 5, the shield conductor 26 c includes anextended area 34, which may increase the size of the shield conductor 26c beyond that of the associated shield dielectric (which is not visiblein FIG. 5). In contrast to other embodiments, in which the shieldingstructure is primarily configured and oriented to overlay or cover thegap 20, the extended area 34 of the shield conductor 26 c is oriented soas not to overlay the gap 20 (or the antenna 16), but rather ispositioned laterally of the antenna 16 and the gap 20, extending awayfrom the antenna 16. The extended area 34 of the shield conductor 26 cmay be variously sized and configured without departing from the scopeof the present disclosure, being approximately the same size as theshield conductor 26 of FIGS. 2A and 2B in one embodiment, larger thanthe shield conductor 26 of FIGS. 2A and 2B in another embodiment, andsmaller than the shield conductor 26 of FIGS. 2A and 2B in yet anotherembodiment.

Regardless of the particular size and configuration of the extended area34 of the shield conductor 26 c, the extended area 34 assists indissipating heat generated across the gap 20. This effect is enhanced byincreasing the size of the extended area 34, so it may be advantageousfor the extended area 34 to be relatively large for improved heatdissipation. The extended area 34 (along with the remainder of theshield conductor 26 c, as well as any of the other shield conductorsdescribed herein) may be formed of a non-flammable material, such as butnot limited to, an aluminum material, heat resistant, flame resistantpaper (Flex Dura HR,http://www.flexlinkllc.com/heat-resistant-paper.html), and non-flammableadhesive (Eclectic E6000 Adhesive,http://eclecticproducts.com/products/e6000.html) to provide a barrier toany arc that may be generated across the gap 20 to prevent a fire fromspreading.

FIGS. 6A and 6B illustrate yet another embodiment of an RFID tag,generally designated at 10 d, (and associated packaging, generallydesignated at 14 d, in FIG. 6B) with a differently configured shieldingstructure 24 d. In the embodiment of FIGS. 6A and 6B, the shielddielectric 28 d is formed of a material which undergoes reversible ornon-reversible dielectric breakdown at high voltages of the type inducedby a high-power microwave field. By such a configuration, the shortingeffect provided by the shielding structure 24 d in the presence of asecond frequency (e.g., in a microwave field) may be enhanced. In thisembodiment (as well as in other embodiments described herein), theshield conductor 26 d may be formed by printing a conductive material(which becomes and defines the shield conductor 26 d) onto the shielddielectric 28 d, such as an over-lamination.

A single RFID tag may include more than one shielding structure, asshown in the embodiment of FIGS. 7A and 7B. In FIG. 7A, the RFID tag,generally designated at 10 e, is provided with a first shieldingstructure, generally designated at 24 e, in general accordance with thepreceding description of the embodiment of FIGS. 3A and 3B. Rather thanthe antenna 16 of the RFID tag 10 e being free for direct connection tothe enclosure of packaging (as in FIG. 3B), a second shieldingstructure, generally designated at 24 f, (FIG. 7B) is associated with anunderside of the antenna 16, with the second shielding structure 24 funderlying the RFID chip 18 (i.e., with the shielding structures 24 eand 24 f electrically coupled to opposing faces of the antenna 16). Theshield dielectric 28 f of the second shielding structure 24 f contactsthe underside of the antenna 16, while the associated shield conductor26 f is free to be secured or otherwise associated to the enclosure of apackage for microwavable food or the like.

In the illustrated embodiment, the second shielding structure 24 f issubstantially identical to the first shielding structure 24 e, but it iswithin the scope of the present disclosure for the shield conductor 26 fand/or the shield dielectric 28 f of the second shielding structure 24 fto be differently configured from the shield conductor 26 e and shielddielectric 28 e of the first shielding structure 24 e. Regardless of theparticular configurations of the two shielding structures 24 e and 24 f,by providing them on both faces of the antenna 16, additional shieldingis provided. This additional shielding involves additional “shorting,”as there are now two partial short circuits across the gap 20. However,in accordance with the preceding description of the embodiment of FIGS.2A and 2B, the antenna 16 is configured so as to compensate for thepresence of the partial short circuits, thereby allowing the RFID tag 10e to operate properly when exposed to the first frequency.

FIG. 8 is a basic equivalent circuit representing the basic componentsof an RFID tag 10 according to the present disclosure. In FIG. 8, thegap 20 defined by the antenna 16 is bridged by an RFID chip 18(represented by a resistor Rp and a capacitor Cp) and a shieldingstructure 24 comprising a shield conductor 26 and a shield dielectric 28(represented by two identical capacitors C_(B) in series). The totalcapacitance of the shield dielectric 28 is half of the capacitance ofthe individual capacitors C_(B) used to represent the shield dielectric28 in FIG. 8. This is calculated using the standard formula in which thetotal capacitance of a series of capacitors is the inverse of the sum ofall inverse capacitances.

The impedance of the shield dielectric 28 is equal to the inverse of theproduct of 2×π×F× total capacitance, in which F is the frequency atwhich the RFID tag 10 is powered. Thus, if the first frequency is on theorder of approximately 800 MHz and the second frequency is on the orderof approximately 2,400 MHz, then impedance drops by a factor ofapproximately three between the first and second frequencies, whichenhances the “shorting” and, hence, shielding effect at the secondfrequency.

Additionally, there is the possibility that an arc may be createdbetween adjacent sections namely gap G and associated RFID chip C. Thisis in part due to adjacent sections being surrounded by a material (i.e.air or other elements) having a dielectric strength lower than that ofthe electric field achieved by said differential voltages across saidadjacent sections. Also an arc may be created and exacerbated in partdue to materials surrounding said sections that reach a temperature, dueto RF current flowing along/through said adjacent sections gap G andchip C, that lowers dielectric strength of the surrounding material aswell as creates flammable/combustible volatiles. This arc can be avoidedwithout the use of a shield by surrounding said sections with a materialhaving the properties such as; a dielectric strength that can withstandthe electric field at said sections, along with having heat resistant,flame resistant and non-flammable properties i.e. heat resistant andflame resistant paper and non-flammable adhesive(s).

Furthermore, within the same scope of the invention additionalembodiments are disclosed. In the illustrated embodiment of FIG. 9, theenclosure 23 is associated with the RFID tag 25 includes an RFID chip 27with an antenna 29 electrically coupled thereto. The antenna 29 isformed of a conductor 31 having a resistance that is greater than theresistance of the antenna 18 of a conventional RFID tag 11, which allowsthe package 21 (including the RFID tag 25) to be safely microwaved. Forexample, the conductor 31 may have a sheet resistance that is comparableto that of the sheet resistance of a susceptor (i.e., in the range ofapproximately 100 ohms to approximately 230 ohms). The conductor 31 mayalso have an optical density in the range of approximately 0.18 to 0.29,similar to a susceptor. By such a configuration, when the RFID tag 25 ismicrowaved, it acts in the way that a susceptor does when beingmicrowaved, by absorbing microwave energy M and heating up andreflecting minimal energy R′, rather than reflecting high levels ofenergy to the microwave source or creating an arc.

The higher sheet resistance of the conductor 31 may affect theefficiency of the antenna 29 compared to the dipole antenna 17 of atypical RFID tag 11. While the sheet resistance of the material(measured in ohms per square at a given thickness) is a fixed value, theresistance experienced by an RF current flowing through the conductor 31may be effectively decreased by increasing the area of the conductor 31(e.g., by increasing its thickness). This is particularly effective inreducing the resistance for an RF current, as skin depth is more of afactor than for a DC current, due to the tendency of an RF current toflow in the outer surface of the conductor 31 (i.e., as conductorthickness is reduced with respect to the skin depth, RF resistancebecomes higher than DC resistance would be). Accordingly, it may beadvantageous for the antenna 29 to have a relatively large area orthickness to decrease the RF resistance.

Compared to a dipole antenna, the conductor of a slot-loop hybridantenna typically has a greater area, such that it may be advantageousfor the antenna 29 to be provided as a slot-loop hybrid antenna(sometimes referred to as a “sloop” antenna), as in FIG. 9. Such aslot-loop hybrid antenna 29 may be formed of a conductor 31 comprising aconductor sheet which, in the illustrated embodiment, is generallyrectangular, with a slot 33 defined therein and positioned at an edge orend 35 of the conductor sheet 31. As shown, the slot 33 may extendbetween a closed end 37 and an open end 39 associated with the end oredge 35 of the conductor sheet 31. While there are various advantages tothe antenna 29 being configured as a slot-loop hybrid antenna, it iswithin the scope of the present disclosure for the antenna 29 to bevariously configured.

Further observing the RFID chip 27, it may take any of a number of forms(including those of the type commonly referred to as a “chip” or a“strap” by one of ordinary skill in the art), including any of a numberof possible components and configured to perform any of a number ofpossible functions. For example, in one embodiment, the RFID chip 27includes an integrated circuit for controlling RF communication andother functions of the RFID tag 25. In the illustrated embodiment, twoends or points of the RFID chip 27 are connected to the conductor sheet31 at opposite sides of the slot 33, adjacent to the open end 39 of theslot 33, which serves to electrically couple the RFID chip 27 to theconductor sheet 31.

According to another aspect of the present disclosure, which may beincorporated into the antenna 29 of the RFID tag 25 of FIG. 9 or may beseparately practiced, an RFID tag 41 (FIGS. 10 and 10A) that is suitablefor incorporation into a package for a microwavable food item may beconfigured to fracture into multiple pieces or otherwise dissociate uponbeing subjected to heating in a microwave oven. By fracturing,interaction with the microwave field is reduced, thereby avoiding thepotential problems of excessive reflected microwave energy and/or thecreation of an arc when the RFID tag 41 is heated in a microwave oven.Such a configuration allows for the resistance of the conductor 43 ofthe antenna 45 of the RFID tag 41 to be lower than in the embodiment ofFIG. 9 (e.g., a sheet resistance of less than 100 ohms), if desired.

The RFID tag 41 shown in FIG. 10 is provided in accordance with theforegoing description of the RFID tag 25 of FIG. 9, with an RFID chip 47electrically coupled to the conductor sheet 43 of a slot-loop hybridantenna 45, although the antenna 45 may be differently configuredwithout departing from the scope of the present disclosure.

Regardless of the particular configuration of the antenna 45, itsconductor sheet 43 is preferably formed of at least two materials (abase material and a secondary material, which may be provided in alesser quantity than the base material) having different coefficients ofthermal expansion. By such a configuration, the materials expand atdifferent rates when heated (e.g., in a microwave oven) until theconductor sheet 43 fractures into multiple pieces or otherwisedissociates. The magnitude of the difference in the coefficients ofthermal expansion of the materials may vary without departing from thescope of the present disclosure, although a relatively large differencemay be advantageous to more quickly cause the conductor sheet 43 tofracture or otherwise dissociate upon heating.

In one exemplary embodiment, the conductor sheet 43 may be formed of abase material, such as a plastic material, and a second material, suchas a metallic material or conductive ink, which have differentcoefficients of thermal expansion. More particularly, the base materialmay be polyethylene terephthalate (which has a coefficient of thermalexpansion of approximately 60 m/(m K)), while the secondary material isaluminum (which has a coefficient of thermal expansion of approximately22 m/(m K)). When bonded together and heated, the aluminum willeventually break, thus rendering the RFID tag 41 inoperative or at leastcausing the RFID tag 41 to operate at a lower level, which reduces theinteraction between the RFID tag 41 and the microwave field. While thebase material has a greater coefficient of thermal expansion than thesecondary material in this example, it is within the scope of thepresent disclosure for the secondary material to have a greatercoefficient of thermal expansion. Furthermore, in one embodiment, thisbreakage may be promoted by including one or more points or lines ofweakness (which are evident in FIG. 10A), such as scored or thinnedareas of decreased thickness, which encourages the conductor sheet 43 tobreak at that particular location or locations.

If it is desired to employ an RFID tag 11 according to conventionaldesign, the manner in which it is incorporated into the package 49 of amicrowavable food item may be modified. FIG. 11 illustrates a package 49incorporating an RFID tag 11 according to conventional design (as inFIG. 1A), although it is also within the scope of the present disclosurefor the RFID tag 11 to be configured as in FIG. 9 or 10.

The enclosure 51 of the package 49 is provided with a joinder material53 applied to one or more of its surfaces (illustrated in FIG. 11 as anouter surface). The joinder material 53 may be present as a relativelythin layer or sheet of material with a resistance that is higher thanthe resistance of the antenna 17 of the RFID tag 11 (e.g., a sheetresistance in the range of approximately 100 ohms to approximately 230ohms). Preferably, the joinder material 53 has a substantially uniformthickness, although it is within the scope of the present disclosure forthe joinder material 53 to have a non-uniform thickness. It may beadvantageous for the joinder material 53 to have an average thicknessthat is less than the thickness of the antenna 17 of the RFID tag 15(e.g., the joinder material 53 may have an average thickness of in therange of approximately 10 nm to approximately 100 nm for joindermaterial 53 comprising an aluminum material).

In one embodiment, the joinder material 53 comprises a metallic film. Inanother embodiment, the joinder material 53 comprises an ink of asuitable conductivity. In other embodiments, the joinder material 53 maybe differently configured, provided that it has a suitably highresistance (i.e., a resistance that is at least greater than theresistance of the antenna 17 of the associated RFID tag 11 and, morepreferably, a sheet resistance in the range of approximately 100 ohms toapproximately 230 ohms).

In the embodiment of FIG. 11, a substrate 55 of the RFID tag 11 (towhich the RFID chip 15 and antenna 17 are mounted) is associated to theenclosure 51 in a manner that sandwiches or interposes the joindermaterial 53 between the RFID tag 11 and the enclosure 51. The joindermaterial 53 itself may have adhesive qualities to cause the RFID tag 11to be secured with respect to the enclosure 51 or a separate means maybe provided to secure the RFID tag 11 to the joinder material 53 (e.g.,an adhesive applied to the underside of the substrate 55). So separatingthe manufacturing of the enclosure 51 with the joinder material 53 andthe RFID tag 11 allows for greater flexibility in manufacturing. Byproviding the joinder material 53 with a relatively high resistance, theeffective sheet resistance of the RFID tag 11 is increased, therebyincreasing the tendency to adsorb RF energy and heat up, rather thancreating an arc.

The joinder material 53 may be variously configured without departingfrom the scope of the present disclosure. For example, the joindermaterial 53 may have a perimeter that substantially coincides with theperimeter of the substrate 55 of the associated RFID tag 11, a perimeterthat extends beyond the entire perimeter of the substrate 55 of theassociated RFID tag 11, a perimeter that is entirely contained withinthe perimeter of the substrate 55 of the associated RFID tag 11, or aperimeter that extends beyond the perimeter of the substrate 55 of theassociated RFID tag 11 in at least one location, while being containedwithin the perimeter of the substrate 55 of the associated RFID tag 11at another location. Additionally, the perimeter of the joinder material53 may have the same shape as the perimeter of the substrate 55 of theassociated RFID tag 11 or a different shape.

In another aspect of the same invention not illustrated, packaging isprovided for a microwavable food item. The packaging includes anenclosure and an RFID tag secured to the enclosure. The RFID tagincludes an antenna defining a gap and configured to operate at a firstfrequency. An RFID chip is electrically coupled to the antenna acrossthe gap. A shielding structure is electrically coupled to the antennaacross the gap and overlays the RFID chip. The shielding structureincludes a shield conductor and a shield dielectric at least partiallypositioned between the shield conductor and the RFID chip. The shieldingstructure is configured to limit the voltage across the gap when theantenna is exposed to a second frequency that is greater than the firstfrequency. The enclosure of the package is provided with the joindermaterial 53 previously described, applied to one or more of its surfaces(similarly illustrated in FIG. 11 as an outer surface). The joindermaterial 53 may be present as a relatively thin layer or sheet ofmaterial with a resistance that is higher than the resistance of theantenna 17 of the RFID tag 11 (e.g., a sheet resistance in the range ofapproximately 100 ohms to approximately 230 ohms).

The present invention also contemplates, but is not limited to, thefollowing testing method for the microwaveable RFID set forth herein.The equipment utilized in one method includes an inverter technologyover such as a 12000 Watts Oven. For instance, a GE® Model JE 2251SJ02can be utilized. Additionally, a scale and a plurality of plasticcontainers to hold samples are used. In one embodiment of the testingmethod, frozen, ground beef was used as a sample. The steps for thetesting method using frozen ground beef are as follows: 1) A sample isprepared. A variety of weights can be utilized. In one instance, a five(5) ounce sample is used. 2) The sample is placed in one half of acontainer in order to ensure that the sample covers the bottom of thecontainer consistently between different tests. 3) The sample is frozenfor approximately twelve (12) hours. 4) At least one RFID label isadhered to the bottom of the container which holds the sample and thesample is placed on a rotational plate within a microwave oven. In oneembodiment, the sample is placed in the center of the rotational platewithin the microwave oven. 5) The sample is microwaved on a full powersetting for two (2) minutes. The present testing method contemplatesthat several different power settings and times can be utilized in orderto test the sample 6) A determination is made as to whether there was a“spark” or “arc.”.

It will be understood that the embodiments described above areillustrative of some of the applications of the principles of thepresent subject matter. Numerous modifications may be made by thoseskilled in the art without departing from the spirit and scope of theclaimed subject matter, including those combinations of features thatare individually disclosed or claimed herein. For these reasons, thescope hereof is not limited to the above description but is as set forthin the following claims, and it is understood that claims may bedirected to the features hereof, including as combinations of featuresthat are individually disclosed or claimed herein.

The invention claimed is:
 1. An RFID tag comprising: an antenna defininga gap and configured to operate at a first frequency; an RFID chipelectrically coupled to the antenna across the gap; and a shieldingstructure electrically coupled to the antenna across the gap, overlayingthe RFID chip, and comprising a shield conductor, and a shielddielectric at least partially positioned between the shield conductorand the RFID chip, wherein the shielding structure is configured tolimit the voltage across the gap when the antenna is exposed to a secondfrequency greater than the first frequency.
 2. The RFID tag of claim 1,wherein the shield dielectric is incorporated into an over-laminationlayer.
 3. The RFID tag of claim 1, wherein the shielding structure isincorporated into an RFID strap.
 4. The RFID tag of claim 1, wherein theshield conductor includes an extended area configured to dissipate heatgenerated across the gap and oriented so as not to overlie the antennaand the gap.
 5. The RFID tag of claim 4, wherein the shield conductor isformed of a non-flammable material.
 6. The RFID tag of claim 1, whereinthe shield dielectric is formed of a material configured to undergo areversible or non-reversible dielectric breakdown when the antenna isexposed to the second frequency.
 7. The RFID tag of claim 1, wherein theshield conductor comprises a conductive material printed onto the shielddielectric.
 8. The RFID tag of claim 1, further comprising a secondshielding structure electrically coupled to the antenna across the gap,underlaying the RFID chip, and comprising a second shield conductor, anda second shield dielectric at least partially positioned between thesecond shield conductor and the antenna.
 9. Packaging for a microwavablefood item comprising: an enclosure; and an RFID tag secured to theenclosure and including an antenna defining a gap and configured tooperate at a first frequency, an RFID chip electrically coupled to theantenna across the gap, and a shielding structure electrically coupledto the antenna across the gap, overlaying the RFID chip, and comprisinga shield conductor, and a shield dielectric at least partiallypositioned between the shield conductor and the RFID chip, wherein theshielding structure is configured to limit the voltage across the gapwhen the antenna is exposed to a second frequency greater than the firstfrequency.
 10. The packaging of claim 9, wherein the shield dielectricis incorporated into an over-lamination layer.
 11. The packaging ofclaim 9, wherein the shielding structure is incorporated into an RFIDstrap.
 12. The packaging of claim 9, wherein the shield conductorincludes an extended area configured to dissipate heat generated acrossthe gap and oriented so as not to overlie the antenna and the gap. 13.The packaging of claim 9, wherein the shield dielectric is formed of amaterial configured to undergo a reversible or non-reversible dielectricbreakdown when the antenna is exposed to the second frequency.
 14. Thepackaging of claim 9, further comprising a second shielding structureelectrically coupled to the antenna across the gap, positioned betweenthe antenna and the enclosure, and comprising a second shield conductor,and a second shield dielectric at least partially positioned between thesecond shield conductor and the antenna.